Eratosthenes was an amazing man in that he was the first person on Earth to measure the circumference of Earth. And despite the basic math and tools that they had such a long time ago, he was only a few thousand miles off! He knew that at noon on June 21st in Syene, the sun shone into a certain well and cast absolutely no shadow on the sides. This meant that the sun was perfectly overhead the well in Syene at noon on June 21st.
Eratosthenes, while back in Alexandria, stuck a vertical pole in the ground on the exact time that the sun cast no shadow on the sides of the well in Syene. The line from the end of the shadow to the top of the pole created a certain angle. He also knew that you could find out how many of those angles were in a circle by dividing 360 degrees by the angle. He then asked the king for his best measured walkers, and measured the distance from Alexandria to Syene. He then multiplied that distance by the times that the angle in Alexandria went into 360 degrees, and thus ended up with the circumference of the earth.
His calculations showed that the earth was between 28,000 and 29,000 miles, while the actual circumference of earth is 24,860 miles.
